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As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal–semiconductor interface at the source and drain electrodes increases, dominating the performance of devices 1–3 . Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS 2 ) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors 4,5 . However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide 3,5–9 . Recent studies have shown that van der Waals contacts formed by transferred graphene 10,11 and metals 12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS 2 . Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS 2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS 2 . The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS 2 and 800 ± 200 ohm micrometres for few-layered MoS 2 . These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS 2 , enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS 2 ) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.
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Acknowledgements M.C., Y.W. and J.Y. acknowledge support from the US National Science Foundation (Civil, Mechanical and Manufacturing Innovation 1727531, Electrical Communications and Cyber Systems 1608389) and Air Force Office of Scientific Research Award FA9550-16-1-0289. M.C. and X.S. acknowledge support from the Shenzhen Peacock Plan (grant number KQTD2016053112042971). J.M. acknowledges support from the Rutgers RiSE summer internship programme. H.Y.J. acknowledges support from the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF-2016M3D1A1900035). R.J.W. and A.M. acknowledge partial support from US NSF MRSEC Award DMR-1420013 for Characterization Facility at the University of Minnesota.
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